Overview

From the PHP script writer's point of view, the change most likely to
impact legacy code is in the way that references are handled in all PHP
versions post-dating the PHP 4.4.0 release.

Until and including PHP 4.3, it was possible to send, assign or return
variables by reference that should really be returned by value, such as
a constant, a temporary value (e.g. the result of an expression), or the
result of a function that had itself been returned by value, as here:

<?php$foo = "123";

function return_value() { global $foo; return $foo;}

$bar = &return_value();?>

Although this code would usually work as expected under PHP 4.3, in the
general case the result is undefined. The Zend Engine could not act
correctly on these values as references. This bug could and did lead to
various hard-to-reproduce memory corruption problems, particularly
where the code base was large.

In PHP 4.4.0, PHP 5.0.4 and all subsequent PHP releases, the Engine was
fixed to 'know' when the reference operation is being used on a value
that should not be referenced. The actual value is now used in such
cases, and a warning is emitted. The warning takes the form of an
E_NOTICE in PHP 4.4.0 and up, and
E_STRICT in PHP 5.0.4 and up.

Code that could potentially produce memory corruption can no longer do
so. However, some legacy code might work differently as a result.

This is because, following the changes, func()
assigns by value. The value of $y is re-assigned,
and reference-binding is preserved from $z. Prior to
the fix, the value was assigned by reference, leading $y
to be re-bound on each assignment. The attempt to bind to a temporary
value by reference was the cause of the memory corruption.

Such code can be made to work identically in both the pre-fix and the
post-fix PHP versions. The signature of func() can
be altered to return by reference, or the reference assignment can be
removed from the result of func().

<?phpfunction func() { return 'function return';}

$x = 'original value';$y =& $x;$y = &func();echo $x;?>

In PHP 4.3 $x would be 'original value', whereas after
the changes it would be 'function return' - remember that where the
function does not return by reference, the reference assignment is
converted to a regular assignment. Again, this can be brought to a common
base, either by forcing func() to return by reference
or by eliminating the by-reference assignment.

Code that worked under PHP 4.3.x, but now throws an error

<?phpclass Foo {

function getThis() { return $this; }

function destroyThis() {$baz =& $this->getThis(); }}

$bar = new Foo();$bar->destroyThis();var_dump($bar);?>

In PHP 5.0.3, $bar evaluated to NULL
instead of returning an object. That happened because
getThis() returns by value, but the value here is
assigned by reference. Although it now works in the expected way, this is
actually invalid code which will throw an E_NOTICE
under PHP 4.4 or an E_STRICT under PHP 5.0.4 and up.

Code that failed under PHP 4.3.x, but now works

In PHP 4.3 the third call to var_dump() produces
NULL, due to the memory corruption caused by
returning an uninitialized value by reference. This is valid code
in PHP 5.0.4 and up, but threw errors in earlier releases of PHP.

Until PHP 5.0.5, it wasn't possible to assign an array element by
reference in this way. It now is.

Code that should have worked under PHP 5.0.x

There are a couple of instances of bugs reported under PHP 5.0 prior to
the reference fixes which now 'work'. However, in both cases errors are
thrown by PHP 5.1.x, because the code was invalid in the first place.
Returning values by reference using self:: now works
in the general case but throws an E_STRICT warning,
and although your mileage may vary when assigning by reference to an
overloaded object, you will still see an E_ERROR
when you try it, even where the assignment itself appears to work.

Warnings that came and went

Nested calls to functions returning by reference are valid code under both
PHP 4.3.x and PHP 5.1.x, but threw an unwarranted
E_NOTICE or E_STRICT under the
intervening PHP releases.